A grease composition is enclosed in a bearing incorporated in various industrial machines and vehicles to impart lubricity to the bearing. The grease composition is prepared by kneading a base oil and a thickener. As the base oil, synthetic oils such as a mineral oil, an ester oil, a silicone oil, an ether oil, a fluorine oil, and the like are generally used. As the thickener, metal soap such as lithium soap, urea compounds, fluororesin are generally used.
In recent years, a rolling bearing tends to be used at a high rotational speed. Thus the grease composition is demanded to have durability at a high temperature. When the metal soap is used as the thickener, oxidation of the base oil is accelerated at the high temperature and thus lubricating action deteriorates. Regarding the base oil, the grease composition containing the mineral oil as the base oil thereof is oxidized more readily than the grease composition containing the synthetic oil as the base oil thereof and thus has a short lubricating life at the high temperature. Thus in recent years, the grease composition containing the synthetic oil as the base oil thereof and the urea compound or the fluororesin as the thickener thereof is mainly used for the rolling bearing which is used at the high temperature and speed.
As various industrial mechanical members become compact and have high performance, use conditions tend to be strict. Consequently the grease composition is demanded to have a higher lubricating performance and a longer lubricating life. In compliance with the demand for the longer lubricating life, the addition of an antioxidant and an antiseptic agent to the grease composition containing a synthetic oil having a high viscosity as its base oil and an urea compound as its thickener is investigated. This grease composition is liable to generate the low-temperature abnormal noise when it is cold.
When a pulley and the like to be driven by an engine of a car is operated when it is cold, there is a case in which an abnormal noise (hoot noise) is generated when it is cold, namely, a low-temperature abnormal noise is generated. Although the reason of the generation of the low-temperature abnormal noise has not been clarified, it is supposed that the low-temperature abnormal noise is generated owing to a self-excited vibration of a rolling element caused by nonuniformity of an oily film of grease. That is, when it is cold, the oily film of the raceway surface is liable to be nonuniform owing to a rise of the viscosity of the base oil of the grease. The oily film causes the coefficient of friction between the rolling element and the raceway surface to make a slight periodical change. Thereby the rolling element generates the self-excited vibration. Owing to the self-excited vibration of the rolling element, the pulley and the like resonate with the bearing. As a result, the outer ring vibrates (translational motion) axially to generate a low-temperature abnormal noise.
As the grease excellent in the durability at the high temperature and preventing the generation of the low-temperature abnormal noise, grease containing mixed oil of synthetic hydrocarbon oil and ester synthetic oil in which not less than eight ester groups are disposed like the teeth of a comb at one side of not less than eight carbon atoms constituting chain-shaped molecules of oil; an urea compound mixed with the base oil as a thickener; and dithiophosphate added to the base oil thereof as an extreme-pressure agent is known (see patent document 1).
A bearing for use in a pulley of a car in which a grease composition containing a base oil composed of a mixed oil of poly-α-olefin (hereinafter abbreviated as PAO) oil and an ester oil; an alicyclic diurea compound mixed with the base oil as a thickener; and zinc dithiocarbamate as an additive added to the base oil is known. In the bearing, a contact angle is formed by bringing each of a plurality of balls interposed between an inner ring and an outer ring both constituting the bearing into contact with the inner ring or with at least the outer ring at two points (see patent document 2).
These attempts are aimed at the stability of the oily film when it is cold to prevent the generation of the low-temperature abnormal noise and at a long life of the grease composition at the high temperature. But these attempts are incapable of obtaining a sufficient effect in preventing the generation of the low-temperature abnormal noise.
Patent document 1: Japanese Patent Application Laid-Open No. 9-208982
Patent document 2: Japanese Patent Application Laid-Open No. 11-270566